Synthesis and characterization of mesoporous silica supported metallosalphen-azobenzene complexes: efficient photochromic heterogeneous catalysts for the oxidation of cyclohexane to produce KA oil

The oxidation of cyclohexane to produce KA oil (cyclohexanone and cyclohexanol) is important industrially but faces challenges such as low cyclohexane conversion at high KA oil selectivity, and difficult catalyst recyclability. This work reports the synthesis and evaluation of new heterogeneous catalysts consisting of Co(ii), Mn(ii), Ni(ii) and Cu(ii) salphen-azobenzene complexes [ML1] immobilized on amino-functionalized mesoporous silica (SBA-15, MCM-41, MCM-48) through coordination bonding. In the first step, the salphen-azobenzene ligand was synthesized and complexed with Co, Mn, Ni and Cu metal ions. In the second step, aminopropyltriethoxysilane (APTES) was grafted onto the surface of different types of commercial mesoporous silica. The immobilization of [ML1] onto the mesoporous silica surface and the thermal stability of the obtained materials were confirmed using different characterization techniques such as FT-IR, powder XRD, SEM, TEM, BET, and TGA. The obtained results revealed high dispersion of [ML1] through the silica surface. The catalytic activity of the prepared materials Silica-N-ML1 was evaluated on the cyclohexane oxidation to produce KA oil using various oxidants. The cis–trans isomerization of the azobenzene upon UV irradiation was found to affect the catalytic performance of Silica-N-ML1. The cis isomer of SBA-15-N-CoL1 exhibited the highest cyclohexane conversion (93%) and KA selectivity (92%) under mild conditions (60 °C, 6 h) using m-CPBA as oxidant. Moreover, The SBA-15-N-CoL1 showed high stability during four successive cycles.


Introduction
The oxidation of cyclohexane is an important industrial chemical reaction.This transformation produces cyclohexanol (A) and cyclohexanone (K), commonly referred to together as ketone-alcohol (KA) oil. 1 KA oil serves as a critical feedstock in the manufacture of nylon 6,6 bers (Scheme 1). 2 The production of nylon 6,6 involves further oxidation of KA oil with nitric acid to form adipic acid.Adipic acid then acts as an important building block monomer in the preparation of nylon 6,6. 3 This nylon polymer nds extensive application in textiles due to their desirable mechanical properties. 4,5Additionally, adipic acid itself is an important intermediate chemical in various industrial processes.][7][8] The current industrial process for the oxidation of cyclohexane to produce KA oil involves cobalt or manganese salt as homogenous catalysts under high-temperature (150-160 °C) and high-pressure conditions (10-20 atm) of air or oxygen gas. 9owever, cyclohexane is quite stable under these conditions while the desired products cyclohexanol and cyclohexanone are less stable, resulting in numerous undesirable by-products forming at the elevated temperatures and pressures. 10Another limitation of this industrial process is that the conversion of cyclohexane must be kept at less than 10% to ensure high selectivity toward KA oil (around 80%). 11Additionally, this process also faces challenges in regenerating and reusing the homogeneous catalysts. 12Therefore, many efforts have been made to develop more efficient catalysts to covert cyclohexane to KA with higher conversion and selectivity under mild conditions. 13ver the past decades, extensive research efforts have been focused on developing heterogeneous catalysts as alternatives to address issues such as low conversion, selectivity, and the high-cost. 14,152][23] Some OTMCs have shown more promise as effective catalysts for the oxidation of cyclohexane. 24Notably, the most extensively investigated OTMCs for cyclohexane oxidation are Schiff base and metallo-porphyrins metal complexes. 22,25][28][29] Salphen, one of the Schiff base ligands, it has proven to be a promising ligand for OTMCs used as catalyst in the oxidation of hydrocarbons, because of its simple synthesis and structural tunability.Salphen are organic compounds with azomethine (-C]N-) groups. 30They are oen made by condensing carbonyl compounds with o-phenylenediamine.The introduction of various metal centres, functional groups, and substituents is made possible by their facile synthesis and structural diversity.This enabled the performance of the obtained catalysts to be optimized for certain reactions. 31Transition metals such as cobalt, copper, manganese, and nickel are of more interest due to their efficiency in the oxidation of hydrocarbons, relative abundance, and lower cost, compared to precious metals such as silver and gold. 32odication of metal-ligand combinations aims to develop sustainable oxidation catalysts.One promising strategy is to insert photoactive azo group into metal-ligand complexes to impart light-responsive properties. 33Azobenzene derivatives have many useful applications.Specically, the cis/trans photoisomerization of azobenzene moiety can nd important utilizations in optics, photochemistry, and biomaterials. 34nserting such photochromic moiety into salphen ligand scaffold could impart new light-responsive functionality to the resulting OTMC catalyst. 35,36However, the effect of cis/trans photoisomerization of the azobenzene on the catalytic activity remains unexplored.Moreover, only few studies have investigated salphen-azobenzene-based OTMCs as catalysts in the oxidation reaction of hydrocarbons. 37Salem and coworkers are the most reported the synthesized and characterization of salphen-azobenzene complexes. 35However, on the best of our knowledge, this type of OTMCs have never been successfully employed as heterogeneous catalysts. 27arious solid supports have been reported for the heterogenization of salphen-based OTMCs.This is including mesoporous silicas, porous carbons, zeolites, polymers, clays and resins. 38][41][42][43] Different techniques have been used to immobilize OTMCs in silica, either onto surface or into framework, such as physical adsorption, graing, co-condensation, periodic mesoporous organosilica (PMOs). 44,45The physical adsorption of OMTCs into silica surface is the easiest technique.However, due to the very weak physical bonds between the catalyst and support, this technique suffers from a major problem, which is the rapid leaching of the catalyst from the support.Previous studies have investigated immobilizing some metal-salphen complexes on solid supports via physical adsorption.5][46][47] While the immobilization of OTMCs in silica through chemical bonding, including graing, cocondensation, PMOs methods, affords more stable OTMCs@silica heterogeneous catalyst. 46,47However, these methods are usually more complicated than the physical approach.

Methods
Proton nuclear magnetic resonance spectrum ( 1 H-NMR) and ( 13 C NMR) of the salphen ligand were acquired in DMSO-d 6 solution using a Brucker AMx 600 MHz spectrometer.Elemental analysis and ICP-mass were used to determine the composition of metallosalphen-azobenzene complexes and the metal content, respectively.The UV/vis spectra of the free salphen ligand and complexation were recorded on a Shimadzu UV-1600 UV/vis spectrophotometer in the wavelength range of 250-700 nm.The morphology of the immobilized catalysts was identied by scanning electron microscopy (SEM; JSM-7100F) (JEOL (Germany) GmbH).Transmission electron microscopy (TEM) micrographs were obtained using an FEI Tecnai G2 F30 TEM operating at 200 kV using a CCD camera.TEM samples were prepared by suspending the material in ethanol using bath sonication for a few minutes, then adding a drop of the resulting suspension solution on carbon coated copper grids with lacey carbon (Ted Pella Inc.) and then letting it dry at room temperature.Important functional groups of salphen ligand complexes and immobilized systems were determined using an FTIR spectrometer (Bruker Vector 22, Ettlingen, Germany) with a wavelength range of 4000 to 500 cm −1 .A Shimadzu Lab-XRD-6000 with CuKa radiation and a secondary monochromator was applied to measure the X-ray diffraction pattern.The thermal stability of the immobilized material was demonstrated under air using a STARe System thermogravimetric analyzer (TGA) operating at a rate of 30 mL min −1 from 25 to 900 °C.The specic surface area, pore volume, pore size, and pore-size distribution of immobilized samples were identied via using a Micrometrics ASAP 2010 apparatus (Norcross, GA).The oxidation reaction was monitored using a Shimadzu GC-2014 gas chromatography (GC) instrument equipped with a ame ionization detector and an FFAP15%CW60/80 column that was 4.0 m long, 0.32 mm in diameter and had a 1 mm lm thickness.Nitrogen was used as the carrier gas at a ow rate of 35 mL min −1 .Samples were withdrawn from the reaction mixture.The injection volume was 1 mL and the total ow rate was 35 mL min −1 .The oven temperature was initially held at 120 °C for 1 minute then increased to 150 °C at 20 °C min −1 and held for 10 minutes.It was then increased to 280 °C at 50 °C min −1 and held for 4 minutes.The injector temperature was 190 °C and the detector temperature was 300 °C.
2.2.1 Measurement of Co, Mn, Ni, and Cu content (%) using ICP-MASS.The metal content in the solid catalysts was determined using inductively coupled plasma mass spectrometry (ICP-MS).The dried catalysts were placed in digestion vessels and treated with a 4 : 1 mixture of hydrochloric acid and nitric acid.The vessels were heated to completely dissolve the catalyst matrices.The resultant digests were then ltered before analysis by ICP-MASS.The ICP-MS was rst calibrated using metal standard solutions to correlate elemental emission intensities to concentration.

Synthesis of salphen azobenzene ligand
The salphen azobenzene ligand N,N 0bis [4-(benzeneazo)salicylaldehyde]-1,2-phenylene-diamine H 2 L 1 was synthesized according to the method reported by Sheykhi-Estalkhjani et al. 51 As outlined in Scheme 2a, the synthesis rst involved the preparation of the azobenzenic intermediate (3a).Aniline (0.10 mol) was dissolved in an aqueous solution of hydrochloric acid to form solution I.In a separate ask, sodium nitrite (0.12 mol) was dissolved in deionized water at 0 °C to yield solution II.Solution II was then added dropwise to solution I at 0 °C, and the resulting diazonium salt mixture was stirred for 30 minutes.Concurrently, salicylaldehyde (0.10 mol) was dissolved in a 10% sodium hydroxide solution (0.10 mol) in another ask maintained at 0 °C.The diazonium salt solution was slowly added to the salicylaldehyde solution.Stirring at 0 °C for 2 h resulted in the precipitation of an orange solid.This precipitate was collected via ltration, dried overnight, and named as compound (3a).Yield: 95%; mp: 185 °C; FT-IR: Ligand H 2 L 1 was synthesized through the condensation between (0.04 mol) of obtained salicylaldehyde azobenzene (3a) and (0.02 mmol) of o-phenylenediamine, using ethanol as solvent (Scheme 2b).The reaction was stirred and reuxed for 6 h.Then the dark yellowish brown precipitated product was cooled down at room temperature, then ltered, rinsed with ethanol, and recrystallized in ethanol.The obtained orange crystals were also ltered, and then dried in air.Yield: 82%; mp:   10.84%; N, 14.34%; O, 5.46%, found C, 65.1%; H, 4.98%; N, 15.3%; O, 10.88%; Co, 8.1% 2.3.3Synthesis of amino-functionalized silica: SBA-15-NH 2 , MCM-41-NH 2 , and MCM-48-NH 2 .Amino-functionalized silica were prepared via a post-synthesis graing method based on the procedure reported by Abboud et al. 52 The synthesis involved dispersing the commercial silica (i.e., SBA-15, MCM-41, or MCM-48) in toluene, followed by dropwise addition of 3-aminopropyltriethoxysilane (APTES).Specically, in 1 L two-neck round-bottom ask equipped with a magnetic stirrer and a reux condenser, 5 g of commercial silica (SBA-15, MCM-41, or MCM-48) was dispersed in 650 mL of toluene, then 5.8 mL (25 mmol) of APTES was added dropwise to silica.Aer agitating the mixture for 15 h at 70 °C, the obtained product was separated by ltration, washed thoroughly with toluene and ethanol (200 mL, 3 times) to remove the excess of APTES.The obtained solid was then dried 2 days at 100 °C.The obtained materials were named as SBA-15-N, MCM-41-N, and MCM-48-N.Co, Mn, Ni or Cu) were obtained aer stirring the mixture at 50 °C for 15 h, followed by ltration and washing thoroughly with CH 2 Cl 2 and EtOH to remove the excess of [ML 1 ], and drying the obtained solid overnight at 80 °C for 15 h.

Oxidation of cyclohexane.
The oxidation of cyclohexane was performed following a procedure reported by Arumugam et al. 53 Briey, 100 mg of the catalyst dispersed in 5 mL of acetonitrile as solvent, 1.0 mL (10 mmol) of CXAN, 1.0 mL (10 mmol) of chlorobenzene as internal standard, and a specic amount of oxidant were added in a autoclave reactor, then the reactor was closed and heated at 60 °C for 6 h.Different oxygen donors were evaluated under these conditions, including hydrogen peroxide (H 2 O 2 ), tert-butyl hydroperoxide (TBHP), and meta-chloroperoxybenzoic acid (m-CPBA).
The conversion of cyclohexane and selectivity of cyclohexanone and cyclohexanol were calculated according to the following equations: where [CXAN] 0 is the initial cyclohexane concentration at time = 0, [CXAN] t is the concentration of cyclohexane aer 6 hours of the reaction.
where [CXON] t is the cyclohexanone concentration aer 6 hours where [CXOL] t is the cyclohexanone concentration aer 6 hours.2.3.6Effect of cis/trans conformation of the catalyst.This study examined the cis/trans isomerization of azobenzene in the catalyst SBA-15-N-CoL 1 .A solution of SBA-15-N-CoL 1 (100 mg) in acetonitrile was placed in an autoclave and irradiated for 45 min with UV light 365 nm.Then, 1.0 mL (10 mmol) of cyclohexane, 1.0 mL (10 mmol) of chlorobenzene, and 2.50 g of m-CBPA were added to the autoclave.The autoclave was sealed, and heated for 6 h at 60 °C.
2.3.7 Catalyst reuse and stability.The recyclability of SBA-15-N-CoL 1 catalyst was tested over multiple runs.Aer each cycle, the catalyst was separated from the reaction mixture by ltration, then dispersed in chloroform for 30 minutes, washing away bound materials.Then the catalyst was isolated by centrifuge from the chloroform.Drying at 70 °C overnight prepared the catalyst for the next cycle under the same conditions.

Synthesis and characterization
FT-IR spectroscopy was used to characterize the structures of the synthesized salphen-azobenzene ligand H 2 L 1 and its Co, Mn, Ni, and Cu complexes.The IR spectrum of H 2 L 1 (Fig. 1a) conrmed the formation of the salphen backbone, due the disappearance of the aldehyde carbonyl peak at 1703 cm −1 (◄) of the azobenzene compound (3a), and appearance of a new imine stretching band at 1606 cm −1 ().Additional characteristic peaks were observed at 1446 cm −1 (D) assigned to the N]N stretching, and 3200 cm −1 (:) corresponding to aromatic C-H stretching.Furthermore, the broad peak at 3447 cm −1 (,) was assigned to O-H stretching.
The coordination of Co(II), Mn(II), Ni(II), and Cu(II) to H 2 L 1 was indicated by shis in the C]N stretching bands to lower wavenumbers of 13-25 cm −1 () in the spectra of [CoL 1 ], [MnL 1 ], [NiL 1 ], and [CuL 1 ] compared to H 2 L 1 (Fig. 1b).The O-H stretching in H 2 L 1 is absent in the complexes, conrming the deprotonation and coordination of O-H to the metal centers.Finally, new weak bands in the low wavenumber region of 678-674 cm −1 (>) and 499-506 cm −1 (-) provide evidence of M-O and M-N bonding in the Co, Mn, Ni, and Cu complexes.Overall, the FT-IR analyses indicated the successful synthesis of the salphen-azobenzene ligand and its Co(II), Mn(II), Ni(II), and Cu(II) metal complexes.
The structure of the synthesized salphen-azobenzene ligand H 2 L 1 was conrmed by the 1 H and 13 C NMR analysis.As shown in Fig. 2, the 1 H NMR spectrum displays a singlet at d 13.23 ppm assigned to the phenolic -OH protons, indicating an enolic contribute to the structure as reported previously. 54Additionally, a characteristic singlet signal observed at d 8.98 ppm attributed to the azo-methine (-CH]N-) protons.The aromatic proton signals are observed at d 6.90-8 ppm. 55The 13 C NMR spectrum presented in Fig. 3  UV-Visible absorption spectra were collected for azobenzene (3a), salphen-azobenzene ligand H 2 L 1 , and its Co, Mn, Ni, and Cu complexes from 260-700 nm in chloroform solution at room temperature, and the obtained results are presented in Fig. 4. The azobenzene intermediate (3a) exhibited characteristic p-p* and n-p* electronic transitions, with the prominent trans p-p* absorption band appearing sharply at 350 nm (Fig. 4a).While the spectrum of ligand H 2 L 1 displayed broader absorption bands that were red-shied, which probably attributed the extension of the conjugated p system.The trans p-p* transition was observed at around 390 nm (Fig. 4a).Upon complexation of    Amino-functionalized materials were prepared by graing aminopropyltriethoxysilane (APTES) onto the surface of SBA-15, MCM-41, and MCM-48, according to literature procedures. 52he FTIR spectra of the commercial silica materials (i.e., SBA-15, MCM-41, and MCM-48), and their corresponding modied silica materials (i.e., SBA-15-N, MCM-41-N, and MCM-48-N) are presented in Fig. 6.In the ngerprint region (700-1300 cm −1 ) of the mesoporous silica spectra, the symmetric (B) and asymmetric (C) stretching vibrations of the Si-O-Si linkages forming the silica frameworks showed two bands at ∼1045 cm −1 and 800 cm −1 , respectively (Fig. 6).Upon graing of APTES onto the silica surfaces, the intensities of both peaks corresponding to the OH stretching (-) and bending (,) vibrations noticeably decreased.This reduction occurred because the silica surface silanol groups (Si-OH) transformed to Si-O-Si-(CH 2 ) 3 -NH 2 aer successful reaction with APTES.Additionally, a new peak at ∼2980 cm −1 (V) appeared, which can be assigned to the CH 2 groups of the graed APTES.Further peaks (>) at ∼700 cm −1 can be attributed to the stretching vibrations of the N-H bond of the APTES.All these ndings demonstrate the successful covalent attachment of APTES onto the surfaces of the silica materials.
Aer the addition of [ML 1 ] complexes to the silica materials, FTIR analysis were performed to the obtained materials Silica-N-ML (i.e., Silica: SBA-15, MCM-41, and MCM-48), and the obtained results are presented in Fig. 7a-c.The intensity of the peaks around 2980-3000 cm −1 (:) corresponding to C-H increased compared to the same peaks in Silica-N.This can be due to the additional peaks of C-H groups in [ML 1 ] complexes.Three peaks were observed at ∼1506 cm −1 (⨀), ∼1400 (C), and ∼1370 (D) which can be assigned to C]N, C]C, and N]N bonds in the metal complexes, respectively.The N-H bending (,) of the NH 2 group was shied to a lower frequency of ∼675 cm −1 aer the addition of [ML 1 ] complexes, which can be due to the formation of a coordination bond between the The average pore diameter and pore volume were derived using the Barrett, Joyner, and Halenda (BJH) method.Fig. 8, 9 and 10 present the resulting Brunauer, Emmett, and Teller (BET) isotherms and the pore size distribution curves of commercial silica (i.e., SBA-15, MCM-41, MCM-48) as well as the aminofunctionalized silica (i.e., SBA-15-N, MCM-41-N, MCM-48-N), and the silica supported metallosalphen-azobenzene complexes Silica-N-ML 1 (M: Co, Mn, Ni, or Cu; Silica: SBA-15, MCM-41, or MCM-48).Moreover, Table 1 summarizes the textural properties of all samples.
The obtained N 2 adsorption/desorption isotherms of all samples demonstrated type IV isotherms with H 1 hysteresis loops, indicating the maintenance of the mesoporous structures aer the amino-functionalization and [ML] complexes immobilization (Fig. 8a).Commercial SBA-15 had a BET surface area of approximately 663.19 m 2 g −1 and pore volume of 0.61 cm 3 g −1 (Table 1).Its pore size distribution curve exhibited a peak at ∼5.6 nm (Fig. 8b).Aer APTES graing, SBA-15-N's surface area and pore volumes decreased to 320.90 m 2 g −1 and 0.46 cm 3 g −1 , respectively, with an average pore diameter of ∼4.8 nm (Fig. 8b), indicating a successful graing of APTES onto SBA-15 surface.
Comparable results were found for the MCM-48 samples.The commercial MCM-48 exhibited a BET surface area of approximately 1000.00 m 2 g −1 , and an average pore volume of 0.56 cm 3 g −1 (Table 1).The pore size distribution of MCM-48 showed a sharp peak at 2.7 nm (Fig. 10b), conrming uniform mesopores.Aer graing APTES onto surface of MCM-48 its surface dramatically decreased to 510 m 2 g −1 .Similarly, the average pore diameter and average pore volume of N-MCM-48 was measured to be approximately 1.6 nm and 0.39 cm 3 g −1 , respectively (Fig. 10b, Table 1).This result indicated the successful graing of APTES onto MCM-48 surface.As expected, aer the incorporation of [ML 1 ] complexes into MCM-48-N the surface area was decreased again to 409 m 2 g −1 , 406 m 2 g −1 , 406   1).In addition, the average pore diameter of the obtained samples were also reduced to 1 nm, 0.98 nm, 0.96 nm, and 0.97 nm respectively (Fig. 10b).Furthermore, similar behavior was observed for the average pore volumes of the prepared samples.The average pore volume of the nal materials was also reduced to 0.23 cm 3 g −1 , 0.23 cm 3 g −1 , 0.20 cm 3 g −1 , and 0.21 cm 3 g −1 , respectively (Table 1).These ndings indicate a successful immobilization of the metal complexes [ML 1 ] onto the amino-functionalized MCM-48-N surface.TGA analysis was conducted on all silica samples, before and aer the incorporation of the metallosalphen-azobenzene complexes to investigate their thermal behavior and determine the [ML 1 ] content in silica.The obtained results are presented in Fig. 11a-c.The thermograms of all samples showed two main steps of weight loss.The initial step below 220 °C presents a weight loss of 2-5%, which is attributed to absorbed solvents and water residues.The second step observed above 220 °C related to the decomposition of the salphen-azobenzene ligand and APTES linker.Notably, the decomposition step of all silica supported [ML 1 ] complexes was started at around 200 °C but extended to around 750 °C, compared to unsupported [ML 1 ] which exhibited a rapid decomposition between 200 °C and 300 °C.This suggests that the silica wall acted as a thermal insulator for the [ML 1 ] complexes.
The thermogravimetric analysis of SBA-15-N, MCM-41-N and MCM-48-N was performed to serve references.For all three materials, the APTES-propylamine groups decomposed between 330-450 °C, with a weight loss of around 6-7%.Specically, SBA-15-N exhibited a weight loss of 4.5% in this temperature range.MCM-41-N showed a similar weight loss of 4.3%.Similar weight loss of 4.7% was also observed for MCM-48-N.The remaining insignicant weight loss occurring up to 850 °C for all materials could be assigned to condensation reactions between silanol groups on the silica surface. 56,57he weight loss observed between 220-750 °C corresponds to the decomposition of the salphen-azobenzene ligand and APTES linker, with organic weight losses (APTES + L) of 13.10% for SBA-15-N-CoL, 13.6% for SBA-15-N-MnL, 12.75% for MCM- 41-N-CoL 1 , 13.5% for MCM-41-N-MnL 1 , 12.94% for MCM-48-N-CoL 1 and 12.01% for MCM-48-N-MnL 1 .This is consistent with the ML 1 content in silica determined to be in the range of 0.97-1.25 wt% as presented in Table 2, which is determined by ICP-MASS.
The morphology of the amino-functionalized silica SBA-15-N, MCM-41-N, and MCM-48-N, along with their corresponding Silica-N-ML samples were investigated using scanning electron microscopy (SEM).The obtained results are presented in Fig. 12.The SEM images SBA-15-N, MCM-41-N, and MCM-48-N (a, f and k), depict particles with length ranging 0.5-5.0mm, 0.10-0.27mm, and 0.12-0.24mm.Their corresponding SBA-15-N-ML 1 (b-e), MCM-41-N-ML 1 (g-j), and MCM-48-N-ML 1 (l-o) depict particles with length ranging (0.5-0.22), (0.29-0.21), and (0.19-0.22) mm respectively.Samples SBA-15-N, MCM-41-N, and MCM-48-N exhibited some degree of aggregation and formation of tubular agglomerates due to formation of hydrogen bonding between the amine groups and the surface silanols.However, aer the addition [ML 1 ] complexes (Fig. 12b-e, g-j and l-o) the aggregation was reduced, and isolated and smaller particles were observed.This decrease provides direct evidence that coordination bonding stabilizes the nanoparticles.Without crosslinking, Silica-N particles could aggregate and grow larger over time.However, the metal-amine coordination bonds restrict this by rigidly connecting particles at a smaller set size.Therefore, the size reductions upon addition of the ML 1 complexes directly support our hypothesis that coordination bonding counteracts aggregative processes and enhances nanoparticle stability.
Transmission electron microscopy (TEM) was used to visualize the nanostructures and dispersion of [ML 1 ] complexes trough silica surface.However, TEM images were obtained only for Silica-N-CoL 1 and Silica-N-MnL 1 and their corresponding silica materials.Because these two catalysts exhibited higher catalytic activity in the oxidation reaction of cyclohexane compared to their Ni and Cu analogues.The obtained TEM images are presented in Fig. 13.Images (a) and (d) display the parallel mesoporous channels, characteristic of the highly nano-ordered SBA-15 and MCM-41 materials, respectively.The TEM images obtained for these two materials aer the incorporation of [CoL 1 ] (Fig. 13b and e) and [MnL 1 ] (Fig. 13c and f) revealed the preservation of the silica nanostructure.More importantly, the absence of any dark spots in the TEM images of Silica-N-CoL 1 and Silica-N-MnL 1 (Silica: SBA-15 or MCM -41)   indicates the high dispersion of [CoL 1 ] and [MnL 1 ] through SBA-15 and MCM-41 surface.
The obtained TEM images showed a spherical morphology of MCM-48 particles (Fig. 13g), with the average particle's diameters of 245 nm.However, images at more than 200 000× were not taken to visualize clearly the very tiny nanochannels of MCM-41 material. 58Aer the addition of [CoL 1 ] and [MnL 1 ] to prepared MCM-4-N-CoL 1 and MCM-4-N-MnL 1 , TEM images (Fig. 13h and i) revealed the preservation of the spherical morphology of MCM-48 with average particles diameter of 220 nm and 222 nm, respectively.Moreover, the absence of any dark spots in the obtained TEM images indicates high dispersion of [CoL 1 ] and [MnL 1 ] through MCM-48 surface.
Powder XRD was performed for the salphen-azobenzene ligand H 2 L 1 and its metal complexes [ML 1 ] where M = Mn, Co, Ni, Cu (Fig. 14).All samples presented distinct peaks corresponding to their crystalline nature.This conrms the formation of metal complexes [ML 1 ].The complexes display less intense peak reections compared to the free ligand which indicates that the crystallinity of the H 2 L 1 decreases upon complexation with M(II) ion = Mn, Co, Ni, and Cu.This observation agrees with previous reports on the impact of metal coordination on ligand crystallinity. 59,60er incorporation of these metal complexes [ML 1 ] into silica Silica-N-ML 1 (Silica: SBA-15, MCM-41 or MCM-48; M: Mn, Co, Ni or Cu), as shown in Fig. 15, all samples displayed a single broad peak around 2q = 22.9°, characteristic of amorphous silica.Notably, no distinctive peaks for crystalline complexes [ML 1 ] were observed.This indicates the metal complexes were highly dispersed on the mesoporous silica surfaces. 61This is in agreement with the TEM results described above.This conrms our approach to prepare Silica-N-ML 1 materials with only isolated [CoL 1 ], [MnL 1 ], [NiL 1 ] and [CuL 1 ] molecules coordinated to surface NH 2 groups, without aggregation phase, and to remove the free molecules by ltration and frequent washing process.

Oxidation of cyclohexane
The catalytic activity of the prepared Silica-N-ML 1 materials was evaluated in the oxidation reaction of cyclohexane to produce KA oil.The reaction was performed in a sealed autoclave.In this study different parameters such as type of oxidant, temperature, reaction time, catalyst dose, solvent.The reaction was monitored by gas chromatography (GC) using chlorobenzene as an internal standard.
3.2.1.3H 2 O 2 .H 2 O 2 was tested as an eco-friendly oxidant, using 2.0 mL (20 mmol) of 30% H 2 O 2 .However, SBA-15-CoL was chosen among the heterogeneous catalysts exhibited the best catalytic activity with m-CPBA and TBHP.The obtained results were compared with some selected results from the literature and presented in Table 5. 21% cyclohexane conversion and 50% KA oil selectivity were obtained with SBA-15-N-CoL 1 as catalyst (entry 1).This result was similar to that obtained with Co-(complex) SiO 2 at 70 °C (entry 2).Better results were obtained by increasing the time or/and temperature (entries 3 and 4).However, even using a cobalt-based catalyst, and increasing the reaction temperature to 100 °C, only 12% of cyclohexane was converted to product, with 80% selectivity (entry 5).
To better understand factors inuencing the catalytic performance of SBA-15-N-CoL 1 , and to optimize the reaction conditions, other parameters were also investigated, such as the reaction temperature, reaction time, catalysts dose, amount of m-CPBA, and cis/trans isomerization of the azobenzene moiety of the ligand H 2 L 1 .
3.2.2Effect of temperature.The effect of temperature on the oxidation reaction of cyclohexane to produce KA oil over SBA-15-CoL 1 was investigated using m-CPBA as oxidant for 6 h, using m-CPBA (1.5 eq., 2.50 g, 15 mmol) as oxidant, with 1.0 mL of cyclohexane (10 mmol), 1.0 mL of chlorobenzene (10 mmol), 100 mg of SBA-15-N-CoL 1 , in 5 mL acetonitrile as a solvent at temperature from RT to 100 °C.The obtained results revealed that cyclohexane conversion was improved when the temperature was increased from room temperature (RT) to 100 °C (Fig. 16).At RT 35% of cyclohexane was converted to products with 32% selectivity toward KA oil.The conversion was gradually increased from 35% to 84% as the temperature increased from RT to 80 °C.However, raised temperature from 80 to 100 °C did not show any signicant change in conversion only a small increment from 84% to 86%, while the selectivity remained at 83% conversion.This in agreement with previous results reported in the literature. 75,76.2.3Effect of reaction time.To optimize the reaction time, the cyclohexane oxidation reaction over SBA-15-CoL 1 was performed in different reaction time, from 0 h to 12 h, using m-CPBA (1.5 eq., 2.50 g mg, 15 mmol) as oxidant, with 1.0 mL of cyclohexane (10 mmol), 1.0 mL of chlorobenzene (10 mmol), 100 mg of SBA-15-N-CoL 1 , in 5 mL acetonitrile as a solvent at 60 °C.As expected, the obtained results revealed that the cyclohexane conversion was gradually increased from 55% to 92% as seen in Fig. 17.However, the selectivity of KA oil initially increased and then remained constant at 89% with longer reaction times.This in agreement with previous results reported in the literature.77 3.2.4Effect of catalysts dose.The effect of catalyst dose was studied over the range of 20-120 mg, using SBA-15-N-CoL 1 as catalyst, m-CPBA (1.5 eq., 2.50 g, 15 mmol) as oxidant, with 1.0 mL of cyclohexane (10 mmol), 1.0 mL of chlorobenzene (10 mmol), in 5 mL acetonitrile as a solvent at 60 °C for 6 h.The obtained results are presented in Fig. 18.The obtained results showed that the cyclohexane conversion and KA oil selectivity were gradually increased from 25% and 25% to 79% and 78% as the catalyst dose increased from 20 to 100 mg, respectively.However, no signicant change was observed in the conversion and selectivity when the catalyst amount increased from 100 mg to 120 mg.Therefore, a catalyst dose of 100 mg was determined to be optimal.
3.2.5Effect of amount of oxidant.In this study, different amounts of m-CPBA (1-2.5 eq.) were used for the oxidation of cyclohexane to produce KA oil over SBA-15-N-CoL to determine the optimized dose.Using 100 mg of catalyst, with 1.0 mL of cyclohexane (10 mmol), 1.0 mL of chlorobenzene (10 mmol), in 5 mL acetonitrile as a solvent at 60 °C for 6 h.The obtained results are seen in Fig. 19.When the amount of m-CPBA was decreased to 1 eq.(10 mmol), both cyclohexane conversion and KA oil selectivity were decreased to 46% and 50%, respectively.Excessively increasing m-CPBA to 2.5 eq.(25 mmol) led to a slight increase in the conversion (86%) and selectivity (79%), compared to result obtained with 1.5 eq. of m-CPBA.
Furthermore, by-products of oxidation of cyclohexane include over-oxidized products resulting from oxidation of the cyclohexanone or cyclohexanol.Common by-products include Table 3 Oxidation of cyclohexane over different catalysts using 1.5 eq.(2.50 g, 15 mmol) m-CPBA as an oxidant.Cyclohexane: 1.0 mL, catalyst: 100 mg, chlorobenzene: 1.0 mL, T: 60 °C, time: 6 h, solvent: 5 mL of acetonitrile cyclohexanone oxime, cyclohexenone, and dicarboxylic acids such as glutaric and adipic acid. 78,79m-CPBA is commonly used as the oxidant for cyclohexane oxidation due to its ability to perform the reaction cleanly with few by-products such as dicarboxylic acid. 3,5m-CPBA is commonly used as the oxidant for cyclohexane oxidation due to its ability to perform the reaction cleanly with few by-products such as dicarboxylic acid. 3,5m-CPBA oxidizes cyclohexane to cyclohexanone and cyclohexanol in a stereospecic catalyst without cleaving the ring.This allows the reaction to obtain high yields of the desired mono-oxidation products with minimal over-oxidation. 80,81The ratio of cyclohexanone to cyclohexanol products (K/A ratio) is affected by several reaction conditions.Previous work has shown that increasing the amount of catalyst, leads to higher conversions but also increases over-oxidation reactions, lowering the K/A ratio. 82Extending the reaction time beyond 7 hours has a similar effect, as longer reactions enhance further oxidation of cyclohexanone and cyclohexanol into by-products. 81,83,84Other studies have also demonstrated the inuence of reaction parameters on cyclohexane oxidation for example, Lesbani and coworker, found that temperatures below 80 °C using m-CPBA resulted in higher cyclohexanone selectivity Table 4 Oxidation of cyclohexane over different catalysts using 1.80 mL (20 mmol) of TBHP as an oxidant.Cyclohexane: 1.0 mL, catalyst: 100 mg, chlorobenzene: 1.0 mL, T: 60 °C, time: 6 h, solvent: 5 mL of acetonitrile Table 5 Oxidation of cyclohexane over different catalysts using 2.0 mL (20 mmol) of 30% H 2 O 2 as an oxidant.Cyclohexane: 1.0 mL, catalyst: 100 mg, chlorobenzene: 1.0 mL, T: 60 °C, time: 6 h, solvent: while also suppressing by-product formation. 76Additionally, Maciuk et al. 2023, showed that the use of an m-CPBA to cyclohexane ratio of 1.5 : 1 improved conversion without signicantly impacting selectivity or yield of by-products. 80ptimal conditions such as a catalyst dose of 100 mg, reaction time between 6-8 hours, and temperature below 80 °C, and a 1.5 : 1 m-CPBA : cyclohexane ratio can minimize by-product formation.
3.2.6Effect of cis/trans conformation of the catalyst.The cis/trans isomerization of the azobenzene in the catalyst SBA-15-N-CoL 1 was conrmed by UV-Visible diffuse reectance (DR) spectroscopy, and the obtained result presented in Fig. 20.Before UV irradiation, the DR spectrum showed the main characteristic absorption band of the trans conguration at 340 nm, which attributed to p-p* transition, with a small band of the cis conguration at 442 nm, which related to n-p* transition (black line).Upon exposure to UV light 365 nm for 45 min, the intensity of the trans band decreased, and cis band increased (green line).This conrms the occurrence of cis/trans isomerization of the azobenzene moiety into SBA-15.
The effect of azobenzene cis/trans isomerization on the catalytic activity of SBA-15-N-CoL 1 was investigated by performing the oxidation reaction of cyclohexane with fresh catalyst and UV irradiated catalyst under the optimized conditions.Actually, in the rst step the catalyst (100 mg) was rst dispersed in acetonitrile (5 mL), then the mixture was exposed to UV light 365 nm for 45 minutes to induce the trans-to-cis isomerization    of the azobenzene groups.In the second step, cyclohexane (1.0 mL, 10 mmol), chlorobenzene (1.0 mL, 10 mmol) as an internal standard, and m-CPBA (2.50 mg, 15 mmol), were added to the catalyst mixture.In the last step, the mixture obtained was poured into a sealed autoclave and heated to 60 °C for 2-8 h.The obtained results (Table 6) revealed that UV irradiation was clearly improved the catalytic activity of SBA-15-N-CoL 1 .For example, the cyclohexane conversion and KA oil selectivity were increased from 86%/85% before the UV irradiation to 93%/92% aer UV irradiation.The high cyclohexane conversion and KA oil selectivity achieved simultaneously over the SBA-15-N-CoL catalysts can be attributed to key features of the photoactive cis-azobenzene complex.Compared to the trans isomer, simulations using density functional theory (DFT) showed the cis conformation offers more accessible active sites for substrate oxidation. 85,86Molecular dynamics simulations further explain that the exible azobenzene ligands in the cis form allow for ideal substrate orientation within the porous framework. 87dditionally, two DFT investigations indicate the photoinduced cis isomer has a narrower HOMO-LUMO gap than trans-azobenzene, consistent with its higher activity in oxidizing cyclohexane. 88These effects are complemented by the exible cis structure enabling dynamic accommodation and orientation of reactant/product molecules, as revealed through experimental kinetic isotope effect measurements. 89The synergistic impact of factors such as the accessible active sites, dynamic substrate positioning, and electronic structure modulation provided by the light-responsive cis complex, helps account for the high conversion and selectivity achieved under mild conditions.This new class of photoactive heterogeneous catalysts extends opportunities for remote performance optimization through photoisomerization. 90he kinetics of the reaction were also studied to determine the catalyst performance.Under the optimized conditions (cyclohexane 1.0 mL (10 mmol), chlorobenzene 1.0 mL, SBA-15-N-CoL 1 100 mg, m-CPBA 2.50 g as an oxidant, acetonitrile 5 mL as a solvent, 60 °C, 6 h), the molar consumption rate of cyclohexane and generation rate of KA oil over SBA-15-N-CoL 1 were determined.The molar consumption of cyclohexane was calculated using the equation: The generation rate of KA oil was calculated using the equation below: KA oil generation rate ¼ ðmoles of KA oil producedÞ=ðmass of catalyst usedÞ Â =ðreaction timeÞ ¼ ð8:56 mmolÞ=ð0:1 gÞ=ð6 hoursÞ ¼ 14:2 mmol g À1 h À1 These kinetic parameters of the SBA-15-N-CoL 1 catalyst conrm its high performance for cyclohexane oxidation.

Catalyst reuse and stability
The recyclability of SBA-15-N-CoL 1 was evaluated through consecutive reaction cycles using the optimized condition.Aer each run, the catalyst was isolated via ltration, washed with chloroform, and dried overnight at 70 °C.The obtained are presented in Fig. 21a.Notably, aer 4 consecutive runs, the  catalyst maintained high catalytic activity, exhibiting only minor decreases in the conversion and selectivity.Moreover, the nanostructure of the recycled catalyst aer the 4 runs was analyzed by TEM and ICP-MS.The characterization outcomes indicated that the structure of the recovered catalyst remained relatively unchanged aer 4 cycles, as evidenced by TEM images (Fig. 21b) and ICP-MS results (1.08 wt%).

Proposed mechanism
Meta-chloroperoxybenzoic acid (m-CPBA) is a widely used and effective organic peroxide that serves as a strong oxidizing agent for many different organic reactions.m-CPBA has been effectively used to activate the typically unreactive C-H bonds found in hydrocarbons. 91The oxidation of C-H bonds is one of the most compelling challenges in organic chemistry.Compared to other common oxidants such as H 2 O 2 and TBHP, m-CPBA exhibits greater stability and selectivity. 92These properties are highly advantageous for organic syntheses.m-CPBA can also form specialized intermediates when used in conjunction with auxiliary reagents, and these intermediates display enhanced reactivity.Moreover, m-CPBA is straightforward to handle as a terminal oxidizing agent. 93,94However, in some cases m-CPBA may non-selectively generate a variety of radical species.Therefore, a catalyst is needed to activate the O-O bond in a targeted manner and control the reaction pathway. 95The catalyst ensures oxidation occurs suitably while suppressing unwanted side and secondary reactions. 96Based on prior theoretical studies of transition metal-catalyzed hydrocarbon oxidation and our experimental results, the following mechanism is proposed over Silica-N-ML catalyst (Scheme 4).

Conclusion
New and efficient photochromic heterogeneous nanocatalysts were successfully synthesized by immobilizing metallosalphenazobenzene complexes onto different mesoporous silica surface.A salphen-azobenzene H 2 L 1 derivative was rs synthetized and complexed with four different transition metals (M: Mn, Co, Ni and Cu).The structure of the obtained complexes [ML 1 ] was conrmed by NMR, IR and elemental analysis and PXRD.Then [ML 1 ] complexes were incorporated into three different pre-prepared amino-functionalized mesoporous silica (N-Silica: SBA-15-N, MCM-41-N, and MCM-48-N) via coordination bonds.The twelve prepared catalysts Silica-N-ML 1 were fully characterized by different techniques such as FT-IR, SEM, TEM, XRD, ICP-MS, DR UV-Vis and N 2 physisorption.The obtained results conrmed the successful graing of APTES and immobilization of [ML 1 ] complexes onto silica surface, with the preservation of the silica mesoporosity and nanostructure order.Results revealed also the presence of trans conguration of the azobenzene group as the major isomer in Silica-N-ML 1 materials, which was easily transformed to cis isomer upon UV irradiation.The catalytic activity of the prepared nanocatalyst (Silica-N-ML 1 ) was evaluated in the oxidation reaction of cyclohexane to produce KA oil.Different parameters were investigated to determine the optimized conditions, such as type of oxidant, type of silica, type of metal, catalyst dose, reaction time, temperature, and UV light.The best results were obtained with 100 mg of SBA-15-N-CoL 1 , using m-CPBA as oxidant, at 60 °C, for 6 h, and under UV light.A superior catalytic activity was observed for the cis conformation under UV light, achieving 93% conversion and 92% selectivity toward KA oil.Moreover, the SBA-15-N-CoL 1 nanocatalyst exhibited a good catalytic activity performance and high stability in four consecutive cycles.Leaching measurement using ICP-MS and TEM images of the spent catalyst conrmed an excellent stability of this photochromic nanocatalyst.
shows the aromatic carbon signals at d 160.20-118.19ppm.The signals observed at d 163.88 and d165.90 ppm correspond to the phenolic C-OH and CH]N carbons, respectively.These ndings conrmed the proposed structure of the synthesized salphen-azobenzene ligand H 2 L 1 .

Table 2
Metal content determined by ICP-MASS a Metal content determined by ICP-MASS.© 2024 The Author(s).Published by the Royal Society of Chemistry RSC Adv., 2024, 14, 26971-26994 | 26983 5 mL of acetonitrile